1. Field of the Invention
This invention relates generally to logic circuits and, more particularly, to a new type of logic circuit family having increased packing density and logic flexibility.
2. Background Art
Known logic circuit families include both unipolar (FET) and bipolar technology. Particularly well known families include resistor-transistor-logic (RTL), diode-transistor logic (DTL), transistor-transistor-logic (TTL), emitter coupled-logic (ECL), integrated-injection-logic (I.sup.2 L), and integrated-Schottky-logic (ISL). The invention described herein most closely relates to I.sup.2 L and ISL; however, advantages over both I.sup.2 L and ISL will be explained in detail below.
Basic I.sup.2 L technology comprises a multi-collector NPN transistor, and a PNP transistor serving as a current source. The collector and base of the current source are connected to the base and emitter of the NPN transistor, respectively. The current source thereby injects minority carriers through the base region of the PNP transistor into the base region of the NPN transistor.
It is readily seen, when monolithically integrated, that the base of the NPN transistor is common to the collector of the current source, while the base of the current source is common to the emitter of the NPN transistor. The emitter of the current source is called the injector. On silicon the entire gate takes up the room of a single multi-emitter transistor.
However, the common base of the current source and the emitter of the NPN transistor is a buried epitaxial layer that is connected to a supply voltage (commonly ground), and requires a contact on top of the chip. Normally, several of these contacts are required due to the resistances involved. These multiple contacts have a negative impact on the density of the chip.
Basic ISL technology comprises a single collector NPN transistor wherein the collector has multiple Schottky contacts thereto. A resistor is generally coupled between the base of the NPN transistor and a supply voltage.
However, the Schottky contacts required for the ISL circuit increases processing complexity, results in a smaller logic swing and has a poor noise immunity. The difference in output voltage between a logical high input signal and a logical low input signal is only about 200 millivolts.
Thus, what is needed is a logic circuit having increased flexibility, increased packing density over I.sup.2 L circuits, and increased noise margin over ISL circuits.